Fibre AOM
A fibre-coupled acousto-optic modulator: a Bragg cell between two fibres. The output fibre takes the diffracted order, shifted in frequency by the drive. It makes a heterodyne interferometer's frequency offset, or a fast switch.
Ports
Ports in and out, and an electrical input, rf.
The model
The sound in the cell, at the drive frequency \(f\), makes a moving grating. The output fibre is aligned to take the order chosen, which is Doppler shifted:
\[\nu_\text{out} = \nu_\text{in} \pm f,\]higher for order +1, lower for −1. The light that is not shifted (order 0) is dumped, and is part of the insertion loss. So
\[\mathbf S_\text{in, out} = 10^{-\ell/20}\sqrt{\eta}\;\mathbf I, \qquad \text{shift } \pm f,\]with \(\ell\) the insertion loss (dB) and \(\eta\) the share of the full drive (1 with no RF input wired). Light going back from out to in is shifted the same way again, so a double pass, through the AOM to a mirror and back, is shifted by \(\pm 2f\), as in a free-space double-pass AOM.
The shifted light makes no steady fringes with the unshifted light of its laser, but beats with it on a photodiode at \(f\) (see Fibre photodetector).
A drive
Wired to the electronics, the RF input sets the share diffracted through time:
\[\eta = \sin^2\!\left(\frac{\pi}{2}\,\frac{v}{V_\text{full}}\right),\]with \(V_\text{full}\) the RF level, in volts, for full efficiency (1 V to begin with). So 0 V passes nothing, and the AOM is an amplitude modulator or a switch.
Settings
| Field | Default | Meaning |
|---|---|---|
rf | 80 | Drive (MHz) |
order | 1 | Order (+1 or −1) |
loss | 2.5 | Insertion loss (dB) |
From the tool's own tables, file format version 1: the names are those in a .jekray file.
Assumptions
- Only the chosen order reaches the output fibre; the coupling into it does not change with the drive frequency.
- There is no rise time: the drive acts at once on the light.